Coherent optical communication technology can provide improved receiver sensitivity compared to direct detection systems in many applications. By amplifying the weak incident signal with a strong local oscillator (LO) output, the system can overcome thermal noise limitations and achieve near quantum-limited sensitivity. In addition, coherent reception offers a better background noise rejection capability since the spectral filtering is performed at the intermediate frequency where the bandwidth can be much more selective. The bandwidth selective nature of the coherent receiver can also lead to a more efficient use of the optical spectrum and the potential of multiple-access communications over a single lasing bandwidth.
In order to realize the full benefits of the coherent system, it is desirable that the transmitted optical signal be phase encoded. Phase encoding provides optimal energy efficiency, and is particularly desirable for deep-space missions where the signal power is at a premium. At the receiving end, the optical signal is coherently detected by spatially mixing the incoming signal with a local oscillator laser output and then detecting it using a balanced detector receiver. Phase modulation of semiconductor lasers can be accomplished by modulating the injection current density and hence the instantaneous frequency of the laser. For CW lasers such as diode pumped solid state lasers an external modulator will be required. Bulk electro-optical (EO) phase modulation often requires high modulation voltage that is not practical to achieve in a flight system. Several approaches can be used to lower the driving voltage requirement. The techniques include travelling wave modulators with long interaction lengths and waveguide modulators with narrow channels (C. M. Gee, G. D. Thurmond and H. W. Yen, "Travelling-Wave Electro-Optic Modulator", Appl. Opt., Vol. 22, No. 13, pp. 2034-2037, July, 1983; I. P. Kaminow, J. R. Carruthers, E. H. Turner, and L. W. Stulz, "Thin-Film LiNbO.sub.3 Electro-Optic Light Modulator", Appl. Phys. Lett., Vol. 22, No. 10, pp. 540-542, May, 1973). Resonant modulators in which the optical and RF modulation signals are both resonated to improve the modulation efficiency have also been proposed and implemented (T. F. Gallagher, N. H. Tran and J. P. Watjen, "Principles of Resonant Cavity Optical Modulator", Appl. Opt., Vol. 25, No. 4, pp. 510-514, Feb. 1986; W. J. Stewart, I. Bennion and M. J. Goodwin, "Electro-Optic Resonant Waveguide Modulation", Tenth European Conference On Optical Communication, Sept. 1984). In order to match the group velocity of optical and RF signals, however, these devices tend to have a narrow modulation bandwidth and cannot be extended to broadband operation needed for data modulation. An alternative is to use only a resonant optical cavity which enhances the interaction length without complex electrode configuration to match the optical and electronic group velocity (W. J. Stewart, I. Bennion and M. J. Goodwin, "Resonant Modulation", Phil. Trans. R. Soc. Lond. A313, p. 401, 1984). Since no electrical resonator is used, the method can in principle be operated at near demodulation frequency. The upper limit of modulation bandwidth is limited by the finesse and hence the cavity lifetime. A resonant ring cavity using this principle has been under investigation for coherent communication (T. J. Kane and R. W. Wallace, "Coherent Communication Link Using Diode-Pumped Lasers", Final Report for Contract NAS5-30487 for NASA Goddard Space Flight Center, August 1989). The present invention relates to an electro-optic resonant phase modulator which as been designed to operate at 100 Mbps. Previously, resonant cavities have also been explored for amplitude modulation (F. R. Nash and P. W. Smith, "Broadband Optical Coupling Modulation", IEEE J. Quantum Electron, Vol. QE-4, pp. 26-34, 1968; D. M. Henderson an V. A. Vilnrotter, "Optical Coupling Modulation In Travelling-Wave Cavities", Appl. Phys. Lett., Vol. 30, No. 7, pp. 335-337, April 1977). The concept of resonant cavity has also been extended to temperature sensors and high-speed signal processing (R.R.A. Syms, "Resonant Cavity Sensor for Integrated Optics", IEEE Journal of Quantum Electronics, Vol QE-21, No. 4, April, 1985) and a passive resonant ring cavity laser gyroscope has been investigated as an alternative to the standard Sagnac interferometer laser gyroscope (K. A. Pugh, "Design, Construction, and Analysis of An Ultra-Low Expansion Quartz Resonant Cavity Passive Ring Resonator Laser Gyroscope", Master of Science Thesis, Air Force Institute of Technology Air University, March 1982).